Hendershot's resting heart rate of 120 beats per minute was abnormally high for someone just relaxing on the couch. A healthy resting heart rate ranges from 60 to 100 beats per minute, according to the American Heart Association.

"At first I thought the watch was wrong because I've always been healthy and I used to be an athlete [...] and I couldn't feel my heart pounding fast which was weird," Hendershot told BuzzFeed News.

To get another non-electronic opinion, she and her husband checked her heart rate manually. The smartwatch was right.

At one point throughout the night, her heart rate reached 160 beats per minute, according to the Topeka Capital-Journal, which first reported the story. The next afternoon, Hendershot visited an urgent care clinic with her husband.

Doctors at the clinic confirmed that her heart rate was abnormally high, but couldn't figure out why. So, she was sent to the emergency room at a nearby hospital where she was diagnosed with severe hyperthyroidism, a condition where the thyroid gland in the neck produces excessive thyroid hormone.

If left untreated, hyperthyroidism can lead to a "thyroid storm," a life-threatening condition in which a person's heart rate, blood pressure, and body temperature can soar to dangerously high levels.

Hendershot was hospitalized and given treatment to slow down or stop her thyroid from producing excess levels of hormones.

Some symptoms of hyperthyroidism include unexpected weight loss, rapid or irregular heartbeat, sweating, irritability, fatigue, anxiety, irregular bowel movements, and more. Other than her irregular heart rate, Hendershot had none of these common symptoms.

Alan Wynne, an endocrinologist at the Cotton O'Neil Diabetes and Endocrinology center at Stormont Vail, who treated Hendershot, told the Topeka Capital-Journal he was "startled and surprised" to hear that the smartwatch's reading was her only symptom.

"My reaction was to smile at her and pause. I asked her twice at first and a third time later, 'Wait a minute, you didn't feel anything?' " he said. "I've been doing this 25 years and it's the first time ever I've heard someone tell me they didn't notice anything and were later diagnosed with severe hyperthyroidism."

Hendershot is thankful her Apple Watch alerted her to the issue. "I felt totally fine. That is why it was so crazy," Hendershot told TODAY. "It saved my life because without it, I wouldn't have known anything was wrong."